1. Field of the Invention
The present invention is directed generally to wireless communication systems and, more specifically, to the transmission of acknowledgement information in an uplink of a communication system.
2. Description of the Art
A communication system includes a DownLink (DL) that conveys transmission signals from a Base Station (BS or NodeB) to User Equipments (UEs) and an UpLink (UL) that conveys transmission signals from UEs to the NodeB. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, etc. A NodeB is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or some other equivalent terminology.
More specifically, the UL conveys data signals carrying information content, control signals providing control information associated with the transmission of data signals on the DL, and Reference Signals (RSs), e.g., pilot signals. The DL also conveys data signals, control signals, and RSs.
UL data signals are conveyed through a Physical Uplink Shared CHannel (PUSCH) and DL data signals are conveyed through a Physical Downlink Shared CHannel (PDSCH).
In the absence of a PUSCH transmission, a UE conveys UL Control Information (UCI) through a Physical Uplink Control CHannel (PUCCH). However, when it has a PUSCH transmission, a UE may convey UCI and data through the PUSCH.
DL control signals may be broadcast or sent in a UE-specific nature. Accordingly, UE-specific control channels can be used, among other purposes, to provide UEs with Scheduling Assignments (SAs) for PDSCH reception, i.e., DL SAs, or PUSCH transmission, i.e., UL SAs. The SAs are transmitted from the NodeB to respective UEs using DL Control Information (DCI) formats through respective Physical DL Control CHannels (PDCCHs).
The NodeB may configure a UE through higher layer signaling, such as Radio Resource Control (RRC) signaling, a PDSCH Transmission Mode (TM), and a PUSCH TM. The PDSCH TM or PUSCH TM is respectively associated with a DL SA or a UL SA and defines whether the respective PDSCH or PUSCH conveys one data Transport Block (TB) or two data TBs.
PDSCH or PUSCH transmissions are either scheduled to a UE by the NodeB through higher layer signaling, such as Radio Resource Control (RRC) signaling, or through physical layer signaling, e.g., a PDCCH, using a respective DL SA or UL SA, or correspond to non-adaptive retransmissions for a given Hybrid Automatic Repeat reQuest (HARQ) process. Scheduling by RRC signaling is semi-static and is referred to as Semi-Persistent Scheduling (SPS). Scheduling by PDCCH is referred to as dynamic. If a UE misses, e.g., fails to decode, a PDCCH, it also misses the associated PDSCH or PUSCH, respectively. This event will be referred to as Discontinuous Transmission (DTX).
The UCI includes ACKnowledgment (ACK) information associated with a HARQ process (HARQ-ACK). The HARQ-ACK information may include multiple bits corresponding to positive acknowledgments (ACKs) for TBs the UE correctly received or negative acknowledgements (NACKs) for TBs the UE incorrectly received. When a UE does not receive a TB, it may transmit DTX (tri-state HARQ-ACK information) or both the absence and the incorrect reception of a TB can be represented by a NACK (in a combined NACK/DTX state). One consequence of a UE not providing DTX information to the NodeB is that the NodeB cannot use Incremental Redundancy (IR) for the HARQ process. This leads to throughput loss. Another consequence is that PDCCH power control, based on DTX feedback, is not possible.
FIG. 1 illustrates a conventional PUSCH transmission structure.
Referring to FIG. 1, a Transmission Time Interval (TTI) is one subframe 110, which includes two slots. Each slot 120 includes NsymbUL symbols, which are used to transmit data signals, UCI signals, or RSs. Each symbol 130 includes a Cyclic Prefix (CP) to mitigate interference due to channel propagation effects. The transmission in one slot may be either at a same or at a different BandWidth (BW) than the transmission in the other slot. Some PUSCH symbols in each slot are used to transmit an RS 140, which enables channel estimation and coherent demodulation of the received data and/or UCI signals.
The transmission BW includes frequency resource units that will be referred to herein as Physical Resource Blocks (PRBs). Each PRB includes NscRB sub-carriers, or Resource Elements (REs), and a UE is allocated MPUSCH PRBs 150 for a total of MscPUSCH=MPUSCH·NscRB for the PUSCH transmission BW.
The last subframe symbol may be used for transmitting a Sounding RS (SRS) 160 from one or more UEs. The SRS provides, to the NodeB, an estimate of the channel medium the respective UE experiences over the SRS transmission BW. The SRS transmission parameters are configured to each UE, by the NodeB, through higher layer signaling.
In FIG. 1, the number of subframe symbols available for transmission of data or UCI signals is NsymbPUSCH=2·(NsymbUL−1)−NSRS, N where NSRS=1 if the last subframe symbol is used for SRS transmission and NSRS=0 otherwise.
FIG. 2 illustrates a conventional transmitter for transmitting data information and HARQ-ACK information in a PUSCH.
Referring to FIG. 2, encoded HARQ-ACK bits 210 and encoded data bits 220 formed into a signal stream by puncturer/inserter 230. Discrete Fourier Transform (DFT) is then performed by DFT unit 240. The REs for the PUSCH transmission BW are selected by the sub-carrier mapping unit 250 as instructed from controller 255. Inverse Fast Fourier Transform (IFFT) is performed by an IFFT unit 260, CP insertion is performed by a CP insertion unit 270, and time windowing is performed by filter 280, thereby generating a transmitted signal 290. For brevity, the encoding and modulation processes and additional transmitter circuitry such as digital-to-analog converter, analog filters, amplifiers, and transmitter antennas are not illustrated.
The PUSCH transmission is assumed to be over a single cluster 295A or over multiple clusters 295B of contiguous REs in accordance to the DFT Spread Orthogonal Frequency Division Multiple Access (DFT-S-OFDMA) method for signal transmission.
FIG. 3 illustrates a conventional receiver for receiving a transmission signal as illustrated in FIG. 2.
Referring to FIG. 3, an antenna receives a Radio-Frequency (RF) analog signal and after further processing, e.g., such filtering, amplification, and analog-to-digital converting, the received digital signal 310 is filtered by filter 320 and the CP is removed by CP removal unit 330. Subsequently, the receiver unit applies a Fast Fourier Transform (FFT) by an FFT unit 340, and selects the REs used by the transmitter by sub-carrier demapping by a sub-carrier demapping unit 350 under a control of controller 355. Thereafter, an Inverse DFT (IDFT) unit 360 applies IDFT, and an extraction unit 370 extracts the HARQ-ACK bits, places erasures at the respective REs for the data bits 380, and finally obtains the data bits 390.
The RS transmission is assumed to be through a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence. Orthogonal multiplexing of CAZAC sequences can be achieved by applying different Cyclic Shifts (CSs) to the same CAZAC sequence.
Assuming, for simplicity, that the PUSCH conveys a single data TB, for HARQ-ACK transmission in a PUSCH, a UE determines a respective number of encoded HARQ-ACK symbols Q′ using Equation (1).
                              Q          ′                =                  min          (                                    ⌈                                                                    O                                          HARQ                      ⁢                                              -                                            ⁢                      ACK                                                        ·                                      β                    offset                                          HARQ                      ⁢                                              -                                            ⁢                      ACK                                                                                                            Q                    m                                    ·                  R                                            ⌉                        ,                          4              ·                              M                sc                PUSCH                                              )                                    (        1        )            
In Equation (1), OHARQ-ACK is a number of HARQ-ACK information bits (HARQ-ACK payload), βoffsetHARQ-ACK is a parameter the NodeB informs to the UE through higher layer signaling, Qm is a number of data information bits per modulation symbol (Qm=2, 4, 6 for QPSK, QAM16, QAM64, respectively), R is a data code rate of an initial PUSCH transmission for the same TB, MscPUSCH is a PUSCH transmission BW in a current subframe, and ┌ ┐ is the ceiling function that rounds a number to its next integer.
The data code rate R is defined as shown in Equation (2).
                    R        =                              (                                          ∑                                  r                  =                  0                                                  CH                  -                  1                                            ⁢                              K                r                                      )                    /                      (                                          Q                m                            ·                              M                sc                                  PUSCH                  ⁢                                      -                                    ⁢                  initial                                            ·                              N                symb                                  PUSCH                  ⁢                                      -                                    ⁢                  initial                                                      )                                              (        2        )            
In Equation (2), CB is a total number of data code blocks and Kr is a number of bits for data code block number r.
The maximum number of encoded HARQ-ACK symbols is limited to the number of REs in 4 DFT-S-OFDM symbols (4·MscPUSCH), which may be located in the two DFT-S-OFDM symbols adjacent to the RS in each of the two subframe slots, as illustrated in FIG. 1.
The determination of the number of encoded HARQ-ACK symbols when a PUSCH conveys multiple TBs, using for example the Single-User Multiple Input Multiple Output (SU-MIMO) transmission method, is similar to when a PUSCH conveys one TB. Accordingly, a repetitive description is omitted for brevity, as it is not material to the present invention.
If a PDSCH conveys one TB, the respective HARQ-ACK information includes one bit, which is encoded as a binary ‘1’, if the TB is correctly received (ACK value), and as a binary ‘0’, if the TB is incorrectly received (NACK value). If a PDSCH conveys two TBs, the HARQ-ACK information includes two bits [o0ACK o1ACK] with o0ACK for TB 0 and o1ACK for TB 1. The transmission of one HARQ-ACK bit may use repetition coding and the transmission of two HARQ-ACK bits may use a (3, 2) simplex code.
A PDSCH with multiple TBs (e.g., two TBs) can be supported using SU-MIMO transmission with a rank greater than 1. In response to a PDSCH reception with multiple TBs, a UE generates multiple respective HARQ-ACK information bits. However, it is possible for a UE to always generate only one HARQ-ACK bit per PDSCH reception by applying HARQ-ACK spatial-domain bundling. With spatial-domain bundling, the HARQ-ACK bit has the NACK value when at least one TB is incorrectly received and has the ACK value when all TBs are correctly received.
For a Time Division Duplex (TDD) communication system, a PDSCH in several DL subframes may be received before a UL subframe is available for HARQ-ACK transmission, either in a PUSCH or in a PUCCH. Then, a UE may need to transmit multiple HARQ-ACK information bits in a single UL subframe. For HARQ-ACK payload OHARQ-ACK>2 bits, a linear block code, such as the (32, OHARQ-ACK) Reed-Mueller code, may be used.
The fundamental conditions for the proper HARQ-ACK transmission in a PUSCH are for the UE and the NodeB to have the same understanding (1) of whether HARQ-ACK information is included in the PUSCH, (2) of the corresponding HARQ-ACK payload, and (3) of how the HARQ-ACK information bits are multiplexed in the PDSCH.
For a Frequency Division Duplex (FDD) communication system and single-cell operation, no specific measures are necessary to ensure the above common understanding, as the communication is over a single cell and the HARQ-ACK information is for a single PDSCH.
In TDD, due to the additional time dimension relative to FDD, a DL Assignment Index (DAI) Information Element (IE), VDAIUL, may be included in UL SAs to inform a UE of the HARQ-ACK payload, if any, to convey in a PUSCH (UL DAI). A DAI IE is also included in DL SAs, VDAIDL, for a UE to determine whether it has missed any DL SA, except for the last DL SA, when the UE transmits HARQ-ACK in the PUCCH (DL DAI). As the NodeB cannot predict whether a given UE will have DL SAs in future DL subframes, the VDAIDL is a relative counter that is incremented in each DL SA transmitted to a given UE and starts from the beginning after the last DL subframe linked to the UL subframe of the HARQ-ACK signal transmission.
FIG. 4 illustrates a conventional PUCCH structure in one subframe slot for a UE to transmit multi-bit HARQ-ACK information based on a DFT-S-OFDM transmission method.
Referring to FIG. 4, after encoding and modulation using, for example, an RM block code and QPSK, respectively, a set of the same HARQ-ACK bits 410 is multiplied 420 with elements of an Orthogonal Covering Code (OCC) 430 and is subsequently DFT precoded 440. For example, for 5 symbols per slot carrying HARQ-ACK bits, the OCC has length 5 {OCC(0), OCC(1), OCC(2), OCC(3), OCC(4)} and can be either of (1, 1, 1, 1, 1), or {1, exp(j2 π/5), exp(j4 π/5), exp(j6 π/5), exp(j8 π/5)}, or {1, exp(j4 π/5), exp(j8 π/5), exp(j2 π/5), exp(j6 π/5)}, or {1, exp(j6 π/5), exp(j2 π/5), exp(j8 π/5), exp(j4 π/5)}, or {1, exp(j8 π/5), exp(j6 π/5), exp(j4 π/5), exp(j2 π/5. The output is passed through an IFFT 450 and it is then mapped to a DFT-S-OFDM symbol 460.
As the previous operations are linear, their relative order may be inter-changed. Because the PUCCH transmission is in 1 PRB, which include NscRB=12 REs, 24 encoded HARQ-ACK bits are transmitted in each slot with QPSK modulation (12 HARQ-ACK QPSK symbols). The same or different HARQ-ACK bits may be transmitted in the second slot of the subframe. In addition to HARQ-ACK signals, RSs are transmitted in each slot to enable coherent demodulation of the HARQ-ACK signals. The RS is constructed from a length-12 CAZAC sequence 470, which passes through an IFFT 480, and is mapped to another DFT-S-OFDM symbol 490.
FIG. 5 illustrates a conventional UE transmitter for HARQ-ACK signals in a PUCCH.
Referring to FIG. 5, the HARQ-ACK information bits 505 are encoded and modulated by an encoder and modulator 510 and then multiplied with an element of the OCC 525 for the respective DFT-S-OFDM symbol by multiplier 520. The output of the multiplier 520 is then precoded by DFT precoder 530. After DFT precoding, sub-carrier mapping is performed by sub-carrier mapper 540, under control of controller 550. Thereafter, the IFFT is performed by IFFT 560, a CP is added by CP inserter 570, and the signal is filtered by filter 580, thereby generating the transmitted signal 590. For brevity, additional transmitter circuitry such as digital-to-analog converter, analog filters, amplifiers, and transmitter antennas are not illustrated in FIG. 5.
FIG. 6 illustrates a conventional NodeB receiver for HARQ-ACK signals in a PUCCH.
Referring to FIG. 6, after receiving an RF analog signal and converting the analog signal to a digital signal 610, the digital signal 610 is filtered by filter 620 and a CP is removed by CP remover 630. Subsequently, the NodeB receiver applies a Fast Fourier Transform (FFT) by FFT 640, performs sub-carrier demapping by sub-carrier demapper 650 under the control of controller 655, and applies an Inverse DFT (IDFT) by IDFT 660. The output of the IDFT 660 is then multiplied with an OCC element 675 for the respective DFT-S-OFDM symbol by multiplier 670. An adder 680 sums the outputs for the DFT-S-OFDM symbols conveying HARQ-ACK signals over each slot, and a demodulator and decoder 690 demodulates and decodes the summed HARQ-ACK signals over both subframe slots to obtain the transmitted HARQ-ACK information bits 695.
The HARQ-ACK signal transmission power in a PUCCH in subframe i, PPUCCH(i), is assumed to be given as shown in Equation (3).PPUCCH(i)=min{PCMAX,h(OHARQ-ACK)+Q(i)}  (3)
In Equation (3), the measure is decibels (dBs) per milliwatt (dBm), PCMAX is the maximum transmission power configured to the UE, Q(i) contains cell-specific and UE-specific parameters, which are not material to the present invention, and h(OHARQ-ACK) is a monotonically increasing function of the HARQ-ACK payload OHARQ-ACK. The derivation of PPUCCH(i) in Equation (3) does not account for possible puncturing of DFT-S-OFDM symbols in a PUCCH, such as, for example, the last symbol to accommodate SRS transmissions, or for possible multiplexing of additional information other than HARQ-ACK in a PUCCH.
For HARQ-ACK transmission in a PUSCH, as the PUSCH transmission power is determined assuming data transmission and as the transmission powers of HARQ-ACK REs and data REs are the same, the HARQ-ACK reception reliability depends on the number of respective REs as reflected in Equation (1). For HARQ-ACK transmission in a PUCCH, the number of REs is predetermined (12 REs over one UL subframe) and the HARQ-ACK reception reliability depends on the transmission power of the HARQ-ACK signal.
In order to increase peak data rates, the NodeB can configure a UE with Carrier Aggregation (CA) of multiple cells to provide higher operating BWs. For example, to support communication over 60 MHz to a UE, CA of three 20 MHz cells can be used. Assuming that the PDSCH in each cell conveys different TBs, the UE generates separate HARQ-ACK information for the respective TBs received in each cell. This is similar to single-cell TDD operation, where the UE generates separate HARQ-ACK information for the respective TBs received in each DL subframe for which the HARQ-ACK transmission is in the same UL subframe.
Unlike the DL SAs in different subframes of a TDD system, for DL CA, the NodeB knows the total number of PDSCH or DL SAs it transmits to a UE in a given subframe. A cell-domain DL DAI IE can then be introduced in DL SAs which, unlike the time-domain DL DAI IE in a TDD system that is a relative counter for the respective DL SA, can be an absolute counter conveying the total number of DL SAs in a given subframe.
A UE is configured with c cells for DL CA. In C2≦C cells, the UE is configured with a PDSCH TM that supports 2 TBs and requires 2 HARQ-ACK information bits. In the remaining C−C2 cells, the UE is configured with a PDSCH TM that supports 1 TB and requires 1 HARQ-ACK information bit. In a given subframe, P≦C PDSCH are transmitted to a UE, with P2≦P PDSCH requiring feedback of 2 HARQ-ACK information bits and P−P2 PDSCH requiring feedback of 1 HARQ-ACK information bit for a total of NTBtransmitted=P+P2 transmitted TBs and a total of P+P2 HARQ-ACK bits. The UE receives R≦P PDSCH with R2≦P2 PDSCH requiring feedback of 2 HARQ-ACK information bits and R−R2 PDSCH requiring feedback of 1 HARQ-ACK information bit for a total of NTBreceived=R+R2 received TBs and a total of R+R2 HARQ-ACK bits. Dynamic switching in a cell between a TM requiring 2 HARQ-ACK information bits and a TM requiring 1 HARQ-ACK information bit is supported.
FIG. 7 illustrates a set of cells configured to a UE, a subset of configured cells with PDSCH transmission in a given subframe from a NodeB to the UE, and a further subset of cells wherein the PDSCH transmission from the NodeB is received by the UE.
Referring to FIG. 7, a UE is configured with C=4 cells, 710, 712, 714, and 716. The PDSCH in C2=2 cells, 710 and 716, conveys 2 TBs. The NodeB transmits DL SAs (and the associated PDSCHs, if any) in P=3 cells, 720, 722, and 726. The NodeB does not transmit DL SA in C−P=1 cell 724. The UE receives DL SAs in R=2 cells, 730 and 732, it does not receive DL SA in one cell 734, and misses the DL SA (and the associated PDSCH, if any) in one cell 736.
For HARQ-ACK transmission in a PUCCH and absence of cell-domain DL DAI IE, because a UE may miss some DL SAs, a common understanding for the HARQ-ACK payload between the UE and the NodeB can be achieved only if the HARQ-ACK payload is always the maximum one of OHARQ-ACK=C+C2 bits (or, with spatial-domain bundling, OHARQ-ACK=C bits). Using the maximum HARQ-ACK payload of OHARQ-ACK=C+C2 bits does not create additional resource overhead. The UE may transmit a NACK or a DTX (in case of tri-state HARQ-ACK information) for the C+C2−R−R2 TBs it did not receive, but the NodeB already knows the cells with no DL SA or PDSCH transmission to the UE and can use the knowledge that the UE transmits a NACK for each of those TBs (a-priori information) to improve the reception reliability of the HARQ-ACK CW. Then, even though the HARQ-ACK payload is OHARQ-ACK=C+C2 bits, the reception reliability of the HARQ-ACK CW is practically the one obtained if the HARQ-ACK payload was P+P2 bits (or, with spatial-domain bundling, P bits).
The NodeB is able to use a-priori information to improve the decoding reliability of the HARQ-ACK CW because a linear block code and QPSK are assumed to be used for the encoding and modulation of the HARQ-ACK bits, respectively. Simply, the NodeB considers only HARQ-ACK CWs having NACK (binary ‘0’) at the predetermined locations as candidates. Due to the implementation of the decoding process, the use of the a-priori information would be impractical or impossible if a convolutional code or a turbo code was used for the encoding or if QAM was used for the modulation of the HARQ-ACK bits.
Although HARQ-ACK signal transmission in a PUCCH with the maximum payload of OHARQ-ACK=C+C2 bits (or, with spatial-domain bundling, of OHARQ-ACK=C bits) does not generate additional resource overhead, it may use larger HARQ-ACK signal transmission power than that needed to achieve the desired reception reliability because the HARQ-ACK payload in the PUCCH is larger than or equal to the actual one of P+P2 bits (or, with spatial-domain bundling, of P bits).
Transmission with larger power than necessary increases UE power consumption and creates additional interference, thereby degrading the reception reliability of signals transmitted in the same BW in other cells.
For HARQ-ACK transmission in a PUSCH, using the maximum HARQ-ACK payload creates unnecessary resource overhead when the actual HARQ-ACK payload is smaller than the maximum one. HARQ-ACK transmission is assumed to be in a single PUSCH, which may be determined, for example, based on a predetermined prioritization for the configured cells.
Therefore, there is a need for deriving the HARQ-ACK payload for transmission in the PUSCH and reducing the corresponding overhead for DL CA.
There is another need for deriving the HARQ-ACK signal transmission power in the PUCCH, while both achieving the desired reception reliability for the HARQ-ACK CW and minimizing interference and UE power consumption for DL CA.
There is another need for setting the appropriate HARQ-ACK transmission power in the PUCCH, while considering the existence of SRS transmission and the multiplexing of other information in the same PUCCH transmission.
There is another need for designing a cell-domain DL DAI IE in the DL SAs and a cell-domain UL DAI IE in the UL SAs, such that the overhead for HARQ-ACK transmission in the PUSCH is minimized, while achieving proper functionality.